1. Field of the Invention
This invention relates to a radio-frequency switch circuit for switching signal paths between an antenna terminal and two or more radio-frequency terminals.
2. Background Art
A multiport radio-frequency switch circuit such as SP6T (single-pole 6-throw) is used, for example, in a multimode, multiband wireless device. In this circuit, multistage series connected through FETs (field effect transistors) are provided between an antenna terminal and each of six radio-frequency terminals, and multistage series connected shunt FETs are provided between each radio-frequency terminal and the ground.
For example, to conduct between the first radio-frequency terminal and the antenna terminal, n-stage connected through FETs between the first radio-frequency terminal and the antenna terminal are turned on, and m-stage series connected shunt FETs between the first radio-frequency terminal and the ground are turned off. At the time, the through FETs between the other radio-frequency terminals and the antenna terminal are all turned off, and all of the shunt FETs between the other radio-frequency terminals and the ground are turned on.
The radio-frequency switch circuit has a problem of harmonic distortion or intermodulation distortion that occurs when a transmitted radio-frequency signal passes through the circuit. This results from nonlinear characteristics of FETs used as switch elements. The distortions can be classified into on-distortion and off- distortion. On-distortion occurs due to the nonlinearity of on-resistance when the FET is in an on-state. And off-distortion occurs due to the nonlinearity of off-capacitance when the FET is in an off-state. The off-distortion is relevant to multiport switches, because one through FET is in the on-state, whereas the number of through FETs in the off-state is the number of ports minus one. In particular, it is important, for UMTS (Universal Mobile Telecommunications System) ports, to suppress third-order intermodulation distortion IMD3 below a system requirements (e.g., −110 dBm).
JP-A-2005-515657 (Kokai) discloses a technique related to a switch circuit and a method for switching radio-frequency signals, which are based on SOI (silicon on insulator) technology and available for wireless applications. This technique proposes an RF switch circuit including a group of a first and second switching transistor and a group of a first and second shunting transistor, and allowing easy integration. However, even using this technique, it is not easy to reduce IMD3 so as to meet the UMTS requirements without increasing a chip size of the radio-frequency switch circuit.